European Patent Document EP 0 371 415 B1 describes a protective relay which is in the form of a thermal overcurrent relay or overload protection relay and is used as a reliable means of protection in overload and phase-failure conditions caused by disconnection of an electrical load, such as an electric motor. In accordance with EP 0 371 415 B1, the essential components of such a thermal protection relay are a bimetal trip element, a transmission mechanism, and an auxiliary switch. In the event of motor overload or phase failure, the bimetal trip element actuates a lever, the movement of which is transmitted by the transmission mechanism to the auxiliary switch. This causes the auxiliary switch to change position. In this manner, the motor is de-energized when exposed to damage and, in addition, the switching state may optionally be signaled via the contacts of the auxiliary switch.
The protective relay of EP 0 371 415 B1 has a current-setting dial which allows the protective relay to be set to different rated motor current values. After an overload trip, the protective relay can be reset either automatically or manually; a push button being provided for selecting between the modes “automatic” and “manual”. In order to manually reset the protective relay after an overload trip, there is provided a reset button which is combined with the mode selection button. Also provided is an OFF/TEST button.
In commercially available protective relays (such as the Siemens Sirius 3R), a protection cap is used to prevent unwanted operation of the current-setting dial and mode selection button. This protection cap is movable and, when in a covering position, covers the current-setting dial and mode selection button, thereby preventing operation of these controls. In order to secure the protection cap in the covering position, or to be able to detect unwanted tampering with the protective relay, a retainer for a security seal is formed integrally with a housing of the protective relay. The seal retainer includes an eye through which a seal wire can be passed. When the protection cap is in the covering position, this eye is in alignment with an eye formed in the protection cap. Once the seal wire is passed through the two eyes and secured by a security seal, the protection cap can no longer be removed from the covering position.
Although in many applications, it is desired to have a tamper protection device including a protection cap and a seal retainer, there is a need for protective relays which are simple and inexpensive in construction and which do not have a tamper protection device. This leads to a multitude of protective relays which differ in construction, resulting in increased manufacturing and handling costs to a supplier of protective relays.